Zebrafish fins provide an excellent model system for revealing the molecular mechanisms controlling size and shape. Fins grow by the addition of bony segments to the distal ends of growing fin rays. Therefore, fin length mutants that produce short segments may identify molecules that regulate bone morphogenesis. Indeed, the short fin mutant produces short fin ray segments, and this phenotype is caused by mutations in zebrafish connexin43. Interestingly, mutations in human connexin43 cause oculodentodigital dysplasia, a syndrome which results in craniofacial and limb malformations. Thus, connexin43 controls the morphology of dermal bone in both zebrafish and humans, and may therefore play similar roles in the regulation of size and shape in these tissues. The underlying goal of this proposal is to correlate the function of connexin43 with the cellular and molecular events of segment morphogenesis. This will be accomplished by four aims. First, several new alleles of connexin43 that modify or limit gap junction intercellular communication will be generated. Second, gap junction communication will be monitored in different alleles of connexin43 in a heterologous assay for cell-cell communication. Third, connexin43 expression will be monitored with respect to the cellular events required for segment morphogenesis, such as cell proliferation and osteoblast differentiation. Finally, the cellular basis for short segments will be examined by monitoring these processes in short fin mutants. Thus, the aims of this proposal will provide immediate insights into the control of bony segment growth in zebrafish. Furthermore, these insights will directly facilitate future understanding of the biology of human bone malformations.